While the involvement of signaling of the serotonin 5HT2A receptor (5HT2AR) and the ADP P2Y1 receptor in the pathogenesis of thrombotic diseases is well documented, there are no antagonists that are currently available for clinical use, and which target either of these pathways. This derives, in part, from lack of knowledge regarding receptor signaling and structure. The present application proposes experiments that address fundamental aspects of the structural biology and signaling of the platelet 5HT2A and P2Y1 receptors: 1. the biological significance of antagonism of 5HT2AR ligand-binding region (i.e., the second extracellular loop; EL2) in the prevention of thrombotic diseases. Our hypothesis is that EL2 of 5HT2AR plays an important role in platelet activation. To address this hypothesis, our experiments will evaluate the ability of a hypothesis is that EL2 of 5HT2AR plays an important role in platelet activation. enhanced, agonist-induced platelet activation. Subsequent studies we will investigate the effects of EL2Ab and an EL2 peptide-based vaccine on bleeding time, and thrombosis development. 2. The G-protein coupling domains of P2Y1 receptor. Our hypothesis is that the intracellular domains of P2Y1 contain separate regions that confine coupling to a specific G-protein. In this regard, one of our interesting resuls is that a peptide mimicking the N-terminus of the second intracellular loop (IL2) of P2Y1 (abbreviated Myr-N-IL2pep) blocked ADP-induced aggregation. This finding suggests that the N-IL2 domain of P2Y1 participates in receptor coupling to Gq. Similarly, the role of other IL regions in G-protein coupling will be determined by examining the effects of their corresponding peptides on ADP-triggered platelet activation. Also, the effects of any biologically-active IL peptides on bleeding time and thrombosis development will be investigated. Collectively, results obtained from these studies will provide fundamental information concerning 5HT2AR and P2Y1 receptor biology and structure, and may define new therapeutic targets and/or aid molecular modeling study predictions for organic derivatives/agents for treating thrombotic disease states.